Isolation of Horizontal Wells
Many wells are drilled horizontally into subterranean formations. On occasion, it is desired to create a number of hydraulic fractures along the length of these horizontal wells. When creating multiple fractures, it is important to be able to isolate one fracture from another so the same part of the well is not repeatedly fractured.
A number of methods have been used to isolate fractures. In one such method, steel pipe (liner) is cemented in the well and then perforated. The liner is first perforated at a first location. A hydraulic fracturing treatment is then performed at this location. After the fracturing treatment, a mechanically set plug (bridge plug) is set inside the casing above the fractured area and a new set of perforations are shot closer to the heel of the well. A second fracturing treatment is then performed at the location of the new set of perforations. The procedure is then repeated along the length of the horizontal section of the liner resulting in a number of perforating and fracturing treatments being completed in the well. While effective at providing isolation between the fractures, this procedure is costly and time consuming.
Another isolation method involves cementing a liner in a horizontal well and perforating the liner at all of the intervals along the well where fracturing is to be performed. An isolation packer assembly is then run on tubing to isolate the various intervals from each other. Fracturing is performed at a perforated interval with the packer cups bounding the area of the fracture. The isolation packer is then pulled up the hole and a fracturing treatment is performed on each perforated interval.
Another isolation method involves leaving the well in an open hole state with no liner or casing in place. In this situation, inflatable packers or other mechanical tools are run in the well to isolate one part of the horizontal well from another. Various fracturing treatments are performed in these isolated intervals between the packers or other tools. Such treatments are performed through tubing.
Another isolation method involves isolating the well with a number of packers and sliding sleeves which are originally run into the well with all the sleeves closed except for the interval closest to the toe of the horizontal well. A fracturing treatment is pumped through this open sleeve. After the treatment, a ball is pumped down the well to isolate the sleeve across the interval that was just fractured and to open a sleeve on a new interval that is closer to the heel of the well. A second fracturing treatment is then pumped into this newly isolated interval. This procedure is repeated on all of the sleeves until all of the intervals in the well have been stimulated.
Another isolation method involves initiating a cut in the formation using a jetting nozzle and sand run on the end of tubing. This cut is immediately followed by a fracturing treatment pumped through the same jetting nozzle or down the annulus of the tubing and the steel casing. This method relies on a negative pressure drop created by the jetting nozzle to divert the fracturing treatment into the cut that was just created in the formation. The tubing is then moved to a new location in the horizontal section of the well. The procedure is repeated. This procedure can be used with tools run on tubing or coiled tubing and can be in cased or open hole. Such methods are described in Canadian Patents 2225571, 2130365, 2104138 and U.S. Pat. Nos. 5,361,856, 5,494,103, 5,765,642 and 7,159,660.
Another method, described in U.S. Pat. No. 4,951,751, involves a method of diverting fracturing treatments in cased wells that are cemented and perforated. In this method, a well is perforated at the farthest desired location from the heel and a fracturing treatment is performed. The fracturing treatment is followed by a solidifiable gel containing a breaker and a solid mechanical wiper plug of rubber, metal, wood, etc. The wiper plug is similar to a pipeline plug. The gel and wiper plug is displaced across the first fracturing treatment and the gel is allowed time to solidify. The well is then perforated again at a location closer to the heel and a second fracturing treatment is performed in a similar manner. The solid wiper plug and the gel both work to divert the fracturing treatment away from the first fracturing treatment. The lack of perforations towards the heel of the fracture ensures diversion of the fracturing treatment above the perforated interval. In this method, the fracturing treatments are pumped directly down the casing with no tubing or coiled tubing in place.
Many wells are drilled vertically or at an angle from vertical (deviated) but are not horizontal. In many of these wells, a number of subterranean formations are present that require hydraulic fracturing treatments. In these wells, it is often desirable to fracture each subterranean formation separately. To do this, a number of methods have been developed to isolate one formation from another.
One such method is to perforate all of the desired intervals and isolate the formations by using a combination of packers and bridge plugs. A subterranean formation is isolated between these mechanical plugs and a fracturing treatment is performed. The plugs and packer are then moved to the next interval and a second fracturing treatment is performed. This procedure is repeated on each formation that needs to be fractured moving up the well until all have been done. In this procedure, the fracturing treatments are normally pumped down tubing but can also be pumped down coiled tubing. The method can also be applied by pumping fracturing treatments down the casing with bridge plugs in place, with no perforations above the interval to be fractured and no packer.
Another method uses rubber cups run on coiled tubing in a perforated cased well. The rubber cups seal to the casing when pressure is applied from the inside and a fracturing treatment is pumped into a formation that is isolated between the cups. When the treatment is completed the pressure is released and the cups relax. The cups are then moved to the next formation and the procedure is repeated. This method is normally used when the fracturing treatment is pumped through coiled tubing but can also be used on regular tubing.
Another method involves a perforation strategy, and on occasion, ball sealers to divert the fracturing treatment into different formations. This method, often known as limited entry fracturing is normally used when the fracturing treatment is pumped directly down the casing with no tubing in the wellbore.